With almost two-thirds of the ballot now counted, Mr Portillo - a self-confessed killer - looks certain to become Guatemala's next president, replacing the outgoing President Alvaro Arzu.

This is Guatemala's first general election since the peace agreement that ended a long-running civil war three years ago.

Mr Portillo's main rival, businessman Oscar Berger of the ruling Party of National Advancement (PAN), has received 31.19% of the vote and left-wing New Nation Alliance (ANN) candidate Alvaro Colom is third with 11.18%.

The winner must receive an outright majority to avoid a second round of voting.

Mike Lanchin: Portillo is set to become Guatemala's fifth president

The BBC's correspondent in the region, Mike Lanchin, says most of the votes pending are from the countryside, where Mr Portillo's populist style of campaigning is expected to have had the best results.

Election officials said more complete results would be delayed until later on Monday because of poor weather and because of unexpectedly high voter turnout.

Some 4.5 million people were registered to elect the president, vice-president, members of Congress and regional assemblies.

Ex-guerrillas take part

Mr Portillo is supported by a former military leader, Rios Montt, whose counter-insurgency campaign 18 years ago was described as genocide by human rights groups.

Latest polls put Oscar Berger in second place

Mr Montt, who is constitutionally barred from running for president because of his past rule, topped a list of congressional candidates for the FRG - a party he helped to found.

Before the election, human rights groups warned Mr Portillo's party represented the country's murky past of political repression and human rights abuses.

But our correspondent says ordinary Guatemalans will have based their choice on how their lives have been affected since President Arzu brought the country's civil war to a formal end with a peace deal with leftist rebels in December 1996.

Although political freedoms and an end to repression are among the more tangible benefits of that peace, it has also brought soaring crime and high unemployment - Mr Portillo has promoted a law and order agenda and supports the death penalty.

Killer

Mr Portillo recently admitted that he, himself, once killed two people at a party 17 years ago, claiming self-defence.

But he has managed to turn the incident into a political asset, arguing that a man capable of defending his life would be certain to defend the lives of Guatemalans.

For his part, Mr Berger has pledged to continue the economic development policies of President Arzu.

The vote comes at a time of growing disillusion over the government's failure to implement key reforms, which were agreed as part of the 1996 peace deal.

These reforms include curbing the military's power, recognising the impoverished Mayan Indian majority and reforming taxation to pay for health, education and housing programmes for the poor.